Screen for x-ray photography



Sept. 6, 1938. K7 WURSTLIN' 2,129,296

SCREEN FOR X-RAY PHOTOGRAPHY Filed May '7, 1936 Karl W6 rsHM I INVENTORBY m ATTORNEY Patented Sept. 6, 1938 UNITED STATES PATENT OFFICE KarlWiirstlin, Berlin, Germany, assignor to Kali- Chemie Aktiengesellschaft,Berlin, Germany Application May 7, 1936, Serial No. 78,397 In GermanyMay 10, 1935 4 Claims.

This invention relates to screens for X-ray (Rontgen ray) photography.As is well known, screens are used in making X-ray pictures in order toreduce the time of exposure. The

5 screens generally consist of a sheet of cardboard or Celluloid to thesurface of which a fluorescent substance such as calcium tungstate isapplied in crystal form and as uniformly as possible by means of abinding agent. The so-called carrierless screens have also beenemployed, which consist only of a layer of gelatine containing calciumtungstate.

An object of the invention is to provide a screen with which X-rayphotographs of much better sharpness can be obtained than with the knownscreens, the time of exposure being the same. A further object of theinvention is to provide a screen which contains the active layer ofcalcium tungstate in the form of a mixture of coarse grains and finergrains.

A further object of the invention is to provide a screen having a layercontaining fluorescent material in which there are arranged in thisactive layer between coarser grains of the fiuores cent material grainsof the same material of considerably smaller diameter and in quantitysuch that the intermediate spaces between the coarser grains are filledfor the most part by the material of finer grain.

Further important features and advantages of the invention will be clearfrom the following description.

It is known that screens containing a layer of calcium tungstate reducethe time of exposure.

The use of these screens has, however, the disadvantage that they havean unfavourable effect upon the sharpness of the photographs owing tothe occurrence of reflected rays. If the attempt be made to obviate thisdisadvantage by treating the layer containing the calcium tungstate witha dyestuif or with a light-absorbing medium or by arranging behind theactive layer a layer which does not substantially reflect the lightemerging from the active layer or by considerably reducing the thicknessof the active layer, then, it is true, an improvement in sharpness isobtained; this improvement is, however, only obtained at the expense ofan increase in the time of exposure, which may be considerable.

It has now been found that the sharpness obtained with the use of ascreen can be considerably increased without the time of exposure beingsimultaneously prolonged by adding to the calcium tungstate used for themanufacture of the known screens or foils a larger or smaller quantityof a calcium tungstate of very fine grain. The very fine-grained calciumtungstate fills the spaces between the coarser crystals. It is clearthat in this way the occurrence of reflected rays is greatly suppressedso that sharper pictures are 5 obtained.

As regards the size of the crystals of the coarsegrained tungstate, theymay vary within wide limits. Thus, for example, material of from 4 to 5or 7 to 8 or 10 can be used. The fine- 10 grained tungstate must possessin comparison with the coarse-grained tungstate a size of grain suchthat it is adapted substantially to fill up the intervening spacesbetween the coarser grains. The best results are obtained when the sizeof 15 grain of the very fine-grained tungstate does not substantiallyexceed 1,11,.

The amount of fine-grained tungstate which is added can vary withinconsiderable limits, for example it may amount to about 20 to 25% of the20 total amount of tungstate. The percentage of very fine-grainedtungstate can, however, be considerably higher, but care should be takenif the best results are to be obtained that so much fine-grainedtungstate of a size of grain which 25 preferably does not considerablyexceed 1 is present that the spaces between the larger crystals arefilled as completely as possible. What proportion is most favourablewhen using a coarser material of a definite size of grain can 30 easilybe determined by making a number of comparative exposures.

The invention is illustrated by way of example. in the accompanyingdrawing which shows an elevated view of a screen according to thepresent 35 invention.

In the drawing I is a carrier sheet consisting of cardboard, Celluloidor the like. 2 designates an active layer of calcium tungstate incrystal form glued tothe carrier sheet by means of a 40 transparentacetyl cellulose lacquer. According to the present invention the activelayer 2 contains calcium tungstate.

The active layer of a screen contains calcium tungstate of a grain sizeof from 4 to 5 and of a 45 grain size of about lit in the ratio of 1:1.When a screen of this kind is used for X-ray exposures, photographs areobtained which are in no way inferior as regards sharpness to thepictures obtained with the use of a screen having a dyed 50 activelayer. At the same time, however, the time required for exposure isconsiderably less and amounts to about half the time required when dyedscreens are employed.

The symbol ,u or micron used in the fore- 55 going specification and inthe following claims to designate the magnitude of the grain size,signifies 0.001 mm.

I claim:-

1. A screen for Rontgen ray exposures in which the active layer contains50% calcium tungstate having a size of grain of from 4 to 5; and 50%calcium tungstate consisting of grains of about and not exceeding 1 insize.

2. A screen for Rontgen ray photographic purposes having a layer offluorescent material,

said layer containing 50 to 80% of the fluorescent material in the formof grains having a size of at" least 4 and the remaining portion offluorescent.

